1. Field of Endeavor
The present invention relates to valve assemblies and, in particular but not exclusively, to Intermediate Pressure (IP) steam valve assemblies incorporating a stop valve and a control valve.
2. Brief Description of the Related Art
As the skilled person will appreciate, a valve between, for example, a steam generator and a turbine, operates in a harsh environment, suffering from both heat and buffeting from the steam flow that it is used to control. Two valves are generally required—a control or ‘governor’ valve, to control the quantity of steam flowing though the turbine (and therefore the speed or load of the turbine) and a ‘stop valve’, which acts in an emergency to stop the steam flow, perhaps to prevent damage to a turbine. At the most basic level, such valves include a plug, or valve head, mounted on a spindle that can be advanced into an operating position partially or wholly blocking a fluid conduit.
More recently, valve assemblies incorporating both stop valves and control valves in one chamber have been developed. In order to house both valves in one chamber, two main options have been adopted as outlined below.
It has been proposed to have two concentric spindles, each carrying a valve that can be advanced separately. Such a valve assembly is described in U.S. Pat. No. 5,971,018, in the name of Karlsson. This valve assembly has the disadvantage that the fast closing or stop valve is actuated by the same fluid that is being controlled. This causes problems in engineering the seals and springs for high temperature and pressure applications. Furthermore, in some embodiments, one spindle is run through another, which is more likely to result in mechanical failure than running a spindle through a static boss, as is established and proven practice.
In a second valve assembly, housing both a stop valve and a control valve, the spindles of each valve are operated from different ends of the assembly using separate actuation mechanisms. Some embodiments seat the stop valve head inside the control valve and, for example, operate the stop valve by pulling and the control valve by pushing. An example of such an assembly is seen in DE-A-2 523 297. However, in such assemblies, one valve spindle is generally in the line of the steam flow, and the duct must turn a corner to avoid its actuator. Every corner in the duct results in lost dynamic pressure of the gas being controlled, which in this case is steam. This can be partly recovered in a diffuser (which is a gradually widening channel) and this is the solution adopted in the prior art. However, such a diffuser is usually made of a costly metal and it is desirable that the use of such material be minimized.